Acute sensitivity and acute tolerance to ethanol in preweanling rats with or without prenatal experience with the drug

The impact of prenatal alcohol, synthetic cannabinoid and co-exposure on behavioral adaptations in adolescent offspring and alcohol self-administration in adulthood

Prenatal exposure to alcohol or cannabinoids can produce enduring neurobiological, cognitive, and behavioral changes in the offspring. Furthermore, prenatal co-exposure to alcohol and cannabinoids induces malformations in brain regions associated with reward and stress-related circuitry. This study examined the effects of co-exposure to alcohol and the synthetic cannabinoid (SCB) CP55,940 throughout gastrulation and neurulation in rats on basal corticosterone levels and a battery of behavioral tests during adolescence and alcohol self-administration in adulthood. Importantly, we find that prenatal alcohol exposure (PAE) caused lower baseline corticosterone levels in adolescent males and females. Co-exposure to alcohol + CP produced hyperactivity during open field test in males, but not females. During the two-bottle choice alcohol-drinking procedure, prenatal cannabinoid exposed male and female adolescent rats drank more alcohol than their vehicle-exposed controls. In adulthood, female rats treated with prenatal cannabinoid exposure (PCE), showed an overall total increase in alcohol intake during alcohol self-administration; but this was not found in males. When the reinforcer was changed to a 1% sucrose solution, male rats exposed to PCE, showed a reduced self-administration compared to vehicle-exposed males, potentially indicative of an anhedonic response. This lower self-administration persisted when 20% alcohol was reintroduced to the sucrose solution. Lastly, following an abstinence period, there were no changes due to prenatal drug exposure in either males or females. Overall, these data suggest lasting consequences of prenatal alcohol and cannabinoid exposure during adolescence and adulthood in male and female rats.

Ethanol's disruptive effects upon early breathing plasticity and blood parameters associated with hypoxia and hypercapnia

Early ethanol exposure affects respiratory neuroplasticity; a risk factor associated with the Sudden Infant Death Syndrome. High and chronic ethanol doses exert long-lasting effects upon respiratory rates, apneic episodes and ventilatory processes triggered by hypoxia. The present study was performed in 3-9-day-old rat pups. Respiratory processes under normoxic and hypoxic conditions were analyzed in pups intoxicated with different ethanol doses which were pre-exposed or not to the drug. A second major goal was to examine if acute and/or chronic early ethanol exposure affects blood parameters related with hypercapnic or hypoxic states. In Experiment 1, at postnatal day 9, animals previously treated with ethanol (2.0 g/kg) or vehicle (0.0 g/kg) were tested sober or intoxicated with 0.75, 1.37 or 2.00 g/kg ethanol. The test involved sequential air conditions defined as initial normoxia, hypoxia and recovery normoxia. Motor activity was also evaluated. In Experiment 2, blood parameters indicative of possible hypoxic and hypercapnic states were assessed as a function of early chronic or acute experiences with the drug. The main results of Experiment 1 were as follows: i) ethanol's depressant effects upon respiratory rates increased as a function of sequential treatment with the drug (sensitization); ii) ethanol inhibited apneic episodes even when employing the lowest dose at test (0.75 g/kg); iii) the hyperventilatory response caused by hypoxia negatively correlated with the ethanol dose administered at test; iv) ventilatory long-term facilitation (LTF) during recovery normoxia was observed in pups pre-exposed to the drug and in pups that received the different ethanol doses at test; v) self-grooming increased in pups treated with either 1.37 or 2.00 g/kg ethanol. The main result of Experiment 2 indicated that acute as well as chronic ethanol exposure results in acidosis-hypercapnia. The results indicate that early and brief experiences with ethanol are sufficient to affect different respiratory plasticity processes as well as blood biomarkers indicative of acidosis-hypercapnia. An association between the LTF process and the acidosis-hypercapnic state caused by ethanol seems to exist. The mentioned experiences with the drug are sufficient to result in an anomalous programming of respiratory patterns and metabolic conditions.

Effect of Modulation of the Astrocytic Glutamate Transporters' Expression on Cocaine-Induced Reinstatement in Male P Rats Exposed to Ethanol

AIM

Reinforcing properties of ethanol and cocaine are mediated in part through the glutamatergic system. Extracellular glutamate concentration is strictly maintained through several glutamate transporters, such as glutamate transporter 1 (GLT-1), cystine/glutamate transporter (xCT) and glutamate aspartate transporter (GLAST). Previous findings revealed that cocaine and ethanol exposure downregulated GLT-1 and xCT, and that β-lactam antibiotics restored their expression.

METHODS

In this study, we investigated the effect of ampicillin/sulbactam (AMP/SUL) (200 mg/kg, i.p.), a β-lactam antibiotic, on cocaine-induced reinstatement and locomotor activity in male alcohol preferring (P) rats using free choice ethanol (15 and 30%, v/v) and water. We also investigated the effect of co-exposure to ethanol and cocaine (20 mg/kg, i.p.) on GLT-1, xCT and GLAST expression in the nucleus accumbens (NAc) core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

RESULTS

Cocaine exposure decreased ethanol intake and preference. Cocaine and ethanol co-exposure acquired place preference and increased locomotor activity compared to ethanol-exposed rats. GLT-1 and xCT expression were downregulated after cocaine and ethanol co-exposure in the NAc core and shell, but not in dmPFC. AMP/SUL attenuated reinstatement to cocaine as well attenuated the decrease in locomotor activity and ethanol intake and preference. These effects were associated with upregulation of GLT-1 and xCT expression in the NAc core/shell and dmPFC. GLAST expression was not affected after ethanol and cocaine co-exposure or AMP/SUL treatment.

CONCLUSION

Our findings demonstrate that astrocytic glutamate transporters within the mesocorticolimbic area are critical targets in modulating cocaine-seeking behavior while being consuming ethanol.

Unraveling the molecular mechanisms involved in alcohol intake and withdrawal in adolescent mice exposed to alcohol during early life stages

Alcohol interferes with foetal development and prenatal alcohol exposure can lead to adverse effects known as foetal alcohol spectrum disorders. We aimed to assess the underlying neurobiological mechanisms involved in alcohol intake and withdrawal in adolescent mice exposed to alcohol during early life stages, in discrete brain areas. Pregnant C57BL/6 female mice were exposed to binge alcohol drinking from gestation to weaning. Subsequently, alcohol seeking and taking behaviour were evaluated in male adolescent offspring, as assessed in the two-bottle choice and oral self-administration paradigms. Brain area samples were analysed to quantify AMPAR subunits GluR1/2 and pCREB/CREB expression following alcohol self-administration. We measured the expression of mu and kappa opioid receptors both during acute alcohol withdrawal (assessing anxiety alterations by the EPM test) and following reinstatement in the two-bottle choice paradigm. In addition, alcohol metabolism was analysed by measuring blood alcohol concentrations under an acute dose of 3 g/kg alcohol. Our findings demonstrate that developmental alcohol exposure enhances alcohol intake during adolescence, which is associated with a decrease in the pCREB/CREB ratio in the hippocampus, prefrontal cortex and striatum, while the GluR1/GluR2 ratio showed a decrease in the hippocampus. Moreover, PLAE mice showed behavioural alterations, such as increased anxiety-like responses during acute alcohol withdrawal, and higher BAC levels. No significant changes were identified for mu and kappa opioid receptors mRNA expression. The current study highlights that early alcohol exposed mice increased alcohol consumption during late adolescence. Furthermore, a diminished CREB signalling and glutamatergic neuroplasticity are proposed as underpinning neurobiological mechanisms involved in the sensitivity to alcohol reinforcing properties.

Learning experiences comprising central ethanol exposure in rat neonates: Impact upon respiratory plasticity and the activity of brain catalase

Fetal ethanol exposure represents a risk factor for sudden infant death syndrome, and the respiratory effects of fetal ethanol exposure promote hypoxic ischemic consequences. This study analyzes central ethanol's effects upon breathing plasticity during an ontogenetic stage equivalent to the human third gestational trimester. Ethanol's unconditioned breathing effects and their intervention in learning processes were examined. Since central ethanol is primarily metabolized via the catalase system, we also examined the effects of early history with the drug upon this system. During postnatal days 3, 5, and 7 (PDs 3-7), pups were intracisternally administered with vehicle or ethanol (300 mg%). They were tested in a plethysmograph scented or not scented with ethanol odor. The state of intoxication attenuated the onset of apneas, a phenomenon that is suggestive of ethanol's anxiolytic effects given the state of arousal caused by the novel environment and the stress of ethanol administration. At PD9, pups were evaluated when sober under sequential air conditions (initial-normoxia, hypoxia, and recovery-normoxia), with or without the presence of ethanol odor. Initial apneic episodes increased when ethanol intoxication was previously associated with the odor. Pups then ingested ethanol, and brain catalase activity was determined. Pre-exposure to ethanol intoxication paired with the odor of the drug resulted in heightened enzymatic activity. Central ethanol exposure appears to exert antianxiety effects that attenuate apneic disruptions. However, during withdrawal, the cues associated with such effects elicit an opposite reaction. The activity of the catalase system was also dependent upon learning processes that involved the association of environmental stimuli and ethanol intoxication.

Co-existence of ethanol-related respiratory and motivational learning processes based on a tactile discrimination procedure in neonatal rats

In rats, high ethanol doses during early postnatal life exert deleterious effects upon brain development that impact diverse social and cognitive abilities. This stage in development partially overlaps with the third human gestational trimester, commonly referred to as the brain growth spurt period. At this stage in development, human fetuses and rat neonates (postnatal days [PD] 3-9) exhibit relatively high respiratory rates that are affected by subteratogenic ethanol doses. Recent studies suggest conditioned breathing responses in the developing organism, given that there are explicit associations between exteroceptive stimuli and the state of ethanol intoxication. Furthermore, studies performed with near-term rat fetuses suggest heightened sensitivity to ethanol's motivational effects. The present study was meant to analyze the unconditioned effects of ethanol intoxication and the possible co-occurrence of learning mechanisms that can impact respiratory plasticity, and to analyze the preference for cues that signal the state of intoxication as well as the effects of the drug, related with motor stimulation. Neonatal rats were subjected to differential experiences with salient tactile cues explicitly paired or not paired with the effects of vehicle or ethanol (2.0 g/kg). A tactile discrimination procedure applied during PDs 3, 5, 7, and 9 allowed the identification of the emergence of ethanol-derived non-associative and associative learning processes that affect breathing plasticity, particularly when considering apneic disruptions. Ethanol was found to partially inhibit the disruptions that appeared to be intimately related with stressful circumstances defined by the experimental procedure. Tactile cues paired with the drug's effects were also observed to exert an inhibitory effect upon these breathing disruptions. The level of contingency between a given tactile cue and ethanol intoxication also resulted in significant changes in the probability of seeking this cue in a tactile preference test. In addition, the state of intoxication exerted motor-stimulating effects. When contrasting the data obtained via the analysis of the different dependent variables, it appears that most ethanol-derived changes are modulated by positive and/or negative (anti-anxiety) reinforcing effects of the drug. As a whole, the study indicates co-existence of ethanol-related functional changes in the developing organism that simultaneously affect respiratory plasticity and preference patterns elicited by stimuli that signal ethanol's motivational effects. These results emphasize the need to consider significant alterations due to minimal ethanol experiences that argue against "safe" levels of exposure in a critical stage in brain development.

Prenatal Alcohol Exposure as a Case of Involuntary Early Onset of Alcohol Use: Consequences and Proposed Mechanisms From Animal Studies

Prenatal alcohol exposure has been found to be an important factor determining later consumption of this drug. In humans, despite the considerable diversity of variables that might influence alcohol consumption, longitudinal studies show that maternal alcohol intake during gestation is one of the best predictors of later alcohol use from adolescence to young adulthood. Experimental studies with animals also provide abundant evidence of the effects of prenatal alcohol exposure on later alcohol intake. In addition to increased consumption, other effects include enhanced palatability and attractiveness of alcohol flavor as well as sensitization to its sensory and reinforcing effects. Most of these outcomes have been obtained after exposing rats to binge-like administrations of moderate alcohol doses during the last gestational period when the fetus is already capable of detecting flavors in the amniotic fluid and learning associations with aversive or appetitive consequences. On this basis, it has been proposed that one of the mechanisms underlying the increased acceptance of alcohol after its prenatal exposure is the acquisition (by the fetus) of appetitive learning via an association between the sensory properties of alcohol and its reinforcing pharmacological effects. It also appears that this prenatal appetitive learning is mediated by the activation of the opioid system, with fetal brain acetaldehyde playing an important role, possibly as the main chemical responsible for its activation. Here, we review and analyze together the results of all animal studies testing these hypotheses through experimental manipulation of the behavioral and neurochemical elements of the assumed prenatal association. Understanding the mechanisms by which prenatal alcohol exposure favors the early initiation of alcohol consumption, along with its role in the causal pathway to alcohol disorders, may allow us to find strategies to mitigate the behavioral effects of this early experience with the drug. We propose that prenatal alcohol exposure is regarded as a case of involuntary early onset of alcohol use when designing prevention policies. This is particularly important, given the notion that the sooner alcohol intake begins, the greater the possibility of a continued history of alcohol consumption that may lead to the development of alcohol use disorders.

Prenatal ethanol exposure modifies locomotor activity and induces selective changes in Met-enk expression in adolescent rats

Several studies suggest that prenatal ethanol exposure (PEE) facilitates ethanol intake. Opioid peptides play a main role in ethanol reinforcement during infancy and adulthood. However, PEE effects upon motor responsiveness elicited by an ethanol challenge and the participation of opioids in these actions remain to be understood. This work assessed the susceptibility of adolescent rats to prenatal and/or postnatal ethanol exposure in terms of behavioral responses, as well as alcohol effects on Met-enk expression in brain areas related to drug reinforcement. Motor parameters (horizontal locomotion, rearings and stereotyped behaviors) in pre- and postnatally ethanol-challenged adolescents were evaluated. Pregnant rats received ethanol (2g/kg) or water during gestational days 17-20. Adolescents at postnatal day 30 (PD30) were tested in a three-trial activity paradigm (habituation, vehicle and drug sessions). Met-enk content was quantitated by radioimmunoassay in several regions: ventral tegmental area [VTA], nucleus accumbens [NAcc], prefrontal cortex [PFC], substantia nigra [SN], caudate-putamen [CP], amygdala, hypothalamus and hippocampus. PEE significantly reduced rearing responses. Ethanol challenge at PD30 decreased horizontal locomotion and showed a tendency to reduce rearings and stereotyped behaviors. PEE increased Met-enk content in the PFC, CP, hypothalamus and hippocampus, but did not alter peptide levels in the amygdala, VTA and NAcc. These findings suggest that PEE selectively modifies behavioral parameters at PD30 and induces specific changes in Met-enk content in regions of the mesocortical and nigrostriatal pathways, the hypothalamus and hippocampus. Prenatal and postnatal ethanol actions on motor activity in adolescents could involve activation of specific neural enkephalinergic pathways.

Behavioral sensitization to ethanol: Neural basis and factors that influence its acquisition and expression

Ethanol-induced behavioral sensitization (EBS) was first described in 1980, approximately 10 years after the phenomenon was described for psychostimulants. Ethanol acts on γ-aminobutyric acid (GABA) and glutamate receptors as an allosteric agonist and antagonist, respectively, but it also affects many other molecular targets. The multiplicity of factors involved in the behavioral and neurochemical effects of ethanol and the ensuing complexity may explain much of the apparent disparate results, found across different labs, regarding ethanol-induced behavioral sensitization. Although the mesocorticolimbic dopamine system plays an important role in EBS, we provide evidence of the involvement of other neurotransmitter systems, mainly the glutamatergic, GABAergic, and opioidergic systems. This review also analyses the neural underpinnings (e.g., induction of cellular transcription factors such as cyclic adenosine monophosphate response element binding protein and growth factors, such as the brain-derived neurotrophic factor) and other factors that influence the phenomenon, including age, sex, dose, and protocols of drug administration. One of the reasons that make EBS an attractive phenomenon is the assumption, firmly based on empirical evidence, that EBS and addiction-related processes have common molecular and neural basis. Therefore, EBS has been used as a model of addiction processes. We discuss the association between different measures of ethanol-induced reward and EBS. Parallels between the pharmacological basis of EBS and acute motor effects of ethanol are also discussed.

Construction of vapor chambers used to expose mice to alcohol during the equivalent of all three trimesters of human development

Exposure to alcohol during development can result in a constellation of morphological and behavioral abnormalities that are collectively known as Fetal Alcohol Spectrum Disorders (FASDs). At the most severe end of the spectrum is Fetal Alcohol Syndrome (FAS), characterized by growth retardation, craniofacial dysmorphology, and neurobehavioral deficits. Studies with animal models, including rodents, have elucidated many molecular and cellular mechanisms involved in the pathophysiology of FASDs. Ethanol administration to pregnant rodents has been used to model human exposure during the first and second trimesters of pregnancy. Third trimester ethanol consumption in humans has been modeled using neonatal rodents. However, few rodent studies have characterized the effect of ethanol exposure during the equivalent to all three trimesters of human pregnancy, a pattern of exposure that is common in pregnant women. Here, we show how to build vapor chambers from readily obtainable materials that can each accommodate up to six standard mouse cages. We describe a vapor chamber paradigm that can be used to model exposure to ethanol, with minimal handling, during all three trimesters. Our studies demonstrate that pregnant dams developed significant metabolic tolerance to ethanol. However, neonatal mice did not develop metabolic tolerance and the number of fetuses, fetus weight, placenta weight, number of pups/litter, number of dead pups/litter, and pup weight were not significantly affected by ethanol exposure. An important advantage of this paradigm is its applicability to studies with genetically-modified mice. Additionally, this paradigm minimizes handling of animals, a major confound in fetal alcohol research.

New evidence of ethanol's anxiolytic properties in the infant rat

Ethanol induces appetitive, aversive, and anxiolytic effects that are involved in the development of ethanol use and dependence. Because early ethanol exposure produces later increased responsiveness to ethanol, considerable effort has been devoted to analysis of ethanol's appetitive and aversive properties during early ontogeny. Yet, there is a relative scarcity of research related to the anxiolytic effects of ethanol during early infancy, perhaps explained by a lack of age-appropriate tests. The main aim of this study was to validate a model for the assessment of ethanol's anxiolytic effects in the infant rat (postnatal days 13-16). The potentially anxiolytic effects of ethanol tested included: i) amelioration of conditioned place aversion, ii) ethanol intake in the presence of an aversive conditioned stimulus, iii) the inhibitory behavioral effect in an anxiogenic environment, and iv) innate aversion to a brightly illuminated area in a modified light/dark paradigm. Ethanol doses employed across experiments were 0.0, 0.5, and 2.0 g/kg. Results indicated that a low ethanol dose (0.5 g/kg) was effective in attenuating expression of a conditioned aversion. Ethanol intake, however, was unaffected by simultaneous exposure to an aversive stimulus. An anxiogenic environment diminished ethanol-induced locomotor stimulation. Finally, animals given 0.5 g/kg ethanol and evaluated in a light/dark box showed increased time spent in the illuminated area and increased latency to escape from the brightly lit compartment than rats treated with a higher dose of ethanol or vehicle. These new results suggest that ethanol doses as low as 0.5 g/kg are effective in ameliorating an aversive and/or anxiogenic state in preweanling rats. These behavioral preparations can be used to assess ethanol's anxiolytic properties during early development.

Prenatal ethanol increases sucrose reinforcement, an effect strengthened by postnatal association of ethanol and sucrose

Late prenatal exposure to ethanol recruits sensory processing of the drug and of its motivational properties, an experience that leads to heightened ethanol affinity. Recent studies indicate common sensory and neurobiological substrates between this drug and sweet tastants. Using a recently developed operant conditioning technique for infant rats, we examined the effects of prenatal ethanol history upon sucrose self-administration (postnatal days, PDs 14-17). Prior to the last conditioning session, a low (0.5 g/kg) or a high (2.5 g/kg) ethanol dose were paired with sucrose. The intention was to determine if ethanol would inflate or devalue the reinforcing capability of the tastant and if these effects are dependent upon prenatal ethanol history. Male and female pups prenatally exposed to ethanol (2.0 g/kg) responded more when reinforced with sucrose than pups lacking this antenatal experience. Independently of prenatal status, a low ethanol dose (0.5 g/kg) enhanced the reinforcing capability of sucrose while the highest dose (2.5 g/kg) seemed to ameliorate the motivational properties of the tastant. During extinction (PD 18), two factors were critical in determining persistence of responding despite reinforcement omission. Pups prenatally exposed to ethanol that subsequently experienced the low ethanol dose paired with sucrose, showed higher resistance to extinction. The effects here reported were not associated with differential blood alcohol levels across prenatal treatments. These results indicate that fetal ethanol experience promotes affinity for a natural sweet reinforcer and that low doses of ethanol are also capable of enhancing the positive motivational consequences of sucrose when ethanol and sucrose are paired during infancy.

Age-dependent effects of stress on ethanol-induced motor activity in rats

RATIONALE

It is important to study age-related differences that may put adolescents at risk for alcohol-related problems. Adolescents seem less sensitive to the aversive effects of ethanol than adults. Less is known of appetitive effects of ethanol and stress modulation of these effects.

OBJECTIVES

This study aims to describe the effects of acute social or restraint stress on ethanol-precipitated locomotor activity (LMA), in adolescent and adult rats. Effects of activation of the kappa system on ethanol-induced LMA were also evaluated.

METHODS

Adolescent or adult rats were restrained for 90 min, exposed to social deprivation stress for 90 or 180 min or administered with the kappa agonist U62,066E before being given ethanol, and assessed for LMA.

RESULTS

Adolescents were significantly more sensitive to the stimulating, and less sensitive to the sedative, effects of ethanol than adults. Basal locomotion was significantly increased by social deprivation stress in adult, but not in adolescent, rats. U62,066E significantly reduced basal and ethanol-induced locomotion in the adolescents. Corticosterone and progesterone levels were significantly higher in adolescents than in adults.

CONCLUSIONS

Adolescents exhibit greater sensitivity to ethanol-induced LMA and reduced sensitivity to ethanol-induced motor sedation than adult rats. Ethanol's effects on motor activity were not affected by acute stress. Unlike adults, adolescents were insensitive to acute restraint and social deprivation stress but exhibited motor depression after activation of the endogenous kappa opioid receptor system.

Prenatal ethanol exposure increases ethanol intake and reduces c-Fos expression in infralimbic cortex of adolescent rats

Prenatal ethanol exposure significantly increases later predisposition for alcohol intake, but the mechanisms associated with this phenomenon remain hypothetical. This study analyzed (Experiment 1) ethanol intake in adolescent inbred WKAH/Hok Wistar rats prenatally exposed to ethanol (2.0g/kg) or vehicle, on gestational days 17-20. Subsequent Experiments (2, 3 and 4) tested several variables likely to underlie the effect of gestational ethanol on adolescent ethanol preference, including ethanol-induced locomotor activation (LMA), ethanol-induced emission of ultrasonic vocalizations (USVs) after exposure to a rough exteroceptive stimulus, and induction of the immediate early gene C-fos in brain areas associated with processing of reward stimuli and with the retrieval and extinction of associative learning. Prenatal ethanol induced a two-fold increase in ethanol intake. Adolescents exhibited significant ethanol-induced LMA, emitted more aversive than appetitive USVs, and postnatal ethanol administration significantly exacerbated the emission of USVs. These effects, however, were not affected by prenatal ethanol. Adolescents prenatally exposed to ethanol as fetuses exhibited reduced neural activity in infralimbic cortex (but not in prelimbic cortex or nucleus accumbens core or shell), an area that has been implicated in the extinction of drug-mediated associative memories. Ethanol metabolism was not affected by prenatal ethanol. Late gestational exposure to ethanol significantly heightened drinking in the adolescent offspring of an inbred rat strain. Ethanol-induced LMA and USVs were not associated with differential ethanol intake due to prenatal ethanol exposure. Prenatal ethanol, however, altered basal neural activity in the infralimbic prefrontal cortex. Future studies should analyze the functionality of medial prefrontal cortex after prenatal ethanol and its potential association with predisposition for heightened ethanol intake.

Prenatal ethanol exposure leads to greater ethanol-induced appetitive reinforcement

Prenatal ethanol significantly heightens later alcohol consumption, but the mechanisms that underlie this phenomenon are poorly understood. Little is known about the basis of 'this effect of prenatal ethanol on the sensitivity to ethanol's reinforcing effects. One possibility is that prenatal ethanol exposure makes subjects more sensitive to the appetitive effects of ethanol or less sensitive to ethanol's aversive consequences. The present study assessed ethanol-induced second-order conditioned place preference (CPP) and aversion and ethanol-induced conditioned taste aversion (CTA) in infant rats prenatally exposed to ethanol (2.0 g/kg) or vehicle (water) or left untreated. The involvement of the κ opioid receptor system in ethanol-induced CTA was also explored. When place conditioning occurred during the ascending limb of the blood-ethanol curve (Experiment 1), the pups exposed to ethanol in utero exhibited greater CPP than untreated controls, with a shift to the right of the dose-response curve. Conditioning during a later phase of intoxication (30-45 min post-administration; Experiment 2) resulted in place aversion in control pups exposed to vehicle during late gestation but not in pups that were exposed to ethanol in utero. Ethanol induced a reliable and similar CTA (Experiment 3) in the pups treated with vehicle or ethanol during gestation, and CTA was insensitive to κ antagonism. These results suggest that brief exposure to a moderate ethanol dose during late gestation promotes ethanol-mediated reinforcement and alters the expression of conditioned aversion by ethanol. This shift in the motivational reactivity to ethanol may be an underlying basis of the effect of prenatal ethanol on later ethanol acceptance.

Ethanol-induced locomotor activity in adolescent rats and the relationship with ethanol-induced conditioned place preference and conditioned taste aversion

Adolescent rats exhibit ethanol-induced locomotor activity (LMA), which is considered an index of ethanol's motivational properties likely to predict ethanol self-administration, but few studies have reported or correlated ethanol-induced LMA with conditioned place preference (CPP) by ethanol at this age. The present study assessed age-related differences in ethanol's motor stimulating effects and analyzed the association between ethanol-induced LMA and conventional measures of ethanol-induced reinforcement. Experiment 1 compared ethanol-induced LMA in adolescent and adult rats. Subsequent experiments analyzed ethanol-induced CPP and conditioned taste aversion (CTA) in adolescent rats evaluated for ethanol-induced LMA. Adolescent rats exhibit a robust LMA after high-dose ethanol. Ethanol-induced LMA was fairly similar across adolescents and adults. As expected, adolescents were sensitive to ethanol's aversive reinforcement, but they also exhibited CPP. These measures of ethanol reinforcement, however, were not related to ethanol-induced LMA. Spontaneous LMA in an open field was, however, negatively associated with ethanol-induced CTA.

Chronic tolerance to the locomotor stimulating effect of ethanol in preweanling rats as a function of social stress

During early stages of development rats are highly sensitive to the locomotor stimulating effect of relatively high ethanol doses, an effect strongly modulated by social stress. This ethanol effect can be modulated by pharmacological treatments that also can attenuate the development of ethanol-induced locomotor sensitization in mice. By the end of the preweanling period the mechanisms underlying sensitization induced by psychostimulants are functional. The aim of the present study was to analyze the locomotor response to ethanol in preweanling rats as a function of repeated exposure to the drug under two different social conditions. Subjects were treated with ethanol between postnatal days 15 and 18 after being isolated for four hours (Experiment 1a) or simply residing in their home-cage (Experiment 1b). After two days of withdrawal locomotor response to ethanol was assessed in both social conditions. In Experiment 2 naïve rats were tested in terms of ethanol-induced activation of the locomotor response in both social conditions. Results from the present study showed no evidence of locomotor sensitization in preweanling rats in any of the social conditions. Instead we observed behavioral tolerance to the stimulating effect of ethanol in animals trained in the home-cage condition, in which subjects trained with ethanol showed sedation in response to ethanol at testing. Overall these results contribute to the understanding of the sensitivity of rats to the acute and chronic locomotor response to ethanol in an ontogenetic period characterized by high sensitivity to ethanol.

Early role of the κ opioid receptor in ethanol-induced reinforcement

Effects of early ethanol exposure on later ethanol intake emphasize the importance of understanding the neurobiology of ethanol-induced reinforcement early in life. Infant rats exhibit ethanol-induced appetitive conditioning and ethanol-induced locomotor activation, which have been linked in theory and may have mechanisms in common. The appetitive effects of ethanol are significantly modulated by μ and δ opioid receptors, whereas μ but not δ receptors are involved in the motor stimulant effects of ethanol during early development. The involvement of the κ opioid receptor (KOR) system in the motivational effects of ethanol has been much less explored. The present study assessed, in preweanling (infant) rats, the modulatory role of the KOR system in several paradigms sensitive to ethanol-induced reinforcement. Kappa opioid activation and blockade were examined in second-order conditioned place preference with varied timing before conditioning and with varied ethanol doses. The role of KOR on ethanol-induced locomotion and ethanol-induced taste conditioning was also explored. The experiments were based on the assumption that ethanol concurrently induces appetitive and aversive effects and that the latter may be mediated by activation of kappa receptors. The main result was that blockade of kappa function facilitated the expression of appetitive ethanol reinforcement in terms of tactile and taste conditioning. The effects of kappa activation on ethanol conditioning seemed to be independent from ethanol's stimulant effects. Kappa opioid activation potentiated the motor depressing effects of ethanol but enhanced motor activity in control subjects. Overall, the results support the hypothesis that a reduced function of the KOR system in nondependent subjects should attenuate the aversive consequences of ethanol.

Early maternal separation affects ethanol-induced conditioning in a nor-BNI insensitive manner, but does not alter ethanol-induced locomotor activity

Early environmental stress significantly affects the development of offspring. This stress has been modeled in rats through the maternal separation (MS) paradigm, which alters the functioning of the HPA axis and can enhance ethanol intake at adulthood. Infant rats are sensitive to ethanol's reinforcing effects, which modulate ethanol seeking and intake. Little is known about the impact of MS on sensitivity to ethanol's appetitive and aversive effects during infancy. The present study assessed ethanol-induced conditioned place preference established through second-order conditioning (SOC), spontaneous or ethanol-induced locomotor activity and ethanol intake in preweanling rats that experienced normal animal facility rearing (AFR) or daily episodes of maternal separation (MS) during postnatal days 1-13 (PDs 1-13). Low-ethanol dose (0.5 g/kg) induced appetitive conditioned place preference (via SOC) in control rats given conventional rearing but not in rats given maternal separation in early infancy, whereas 2.0 g/kg ethanol induced aversive conditioned place preference in the former but not the latter. The administration of a kappa antagonist at PD 1 or immediately before testing did not alter ethanol-induced reinforcement. High (i.e., 2.5 and 2.0 g/kg) but not low (i.e., 0.5 g/kg) ethanol dose induced reliable motor stimulation, which was independent of early maternal separation. Ethanol intake and blood alcohol levels during conditioning were unaffected by rearing conditions. Pups given early maternal separation had lower body weights than controls and showed an altered pattern of exploration when placed in an open field. These results indicate that, when assessed in infant rats, earlier maternal separation alters the balance between the appetitive and aversive motivational effects of ethanol but has no effect on the motor activating effects of the drug.

Mianserin, but not ondansetron, reduces the locomotor stimulating effect of ethanol in preweanling rats

During infancy rats are highly sensitive to the locomotor stimulating effect of ethanol, an effect particularly observed when they are tested during the rising phase of the blood ethanol curve and in a novel environment. According to a recent study infant rats require some degree of stress to get stimulated after being challenged with ethanol. Ethanol-induced stimulation in preweanling rats required the activation of CRH-1 receptors. Considering these antecedents, we explored modulation of the acute stimulating effect of ethanol (2.5g/kg) by two anxiolytic drugs, Mianserin (2.5 or 5mg/kg) and Ondansetron (1 or 3mg/kg). Mianserin attenuated the stimulating effect of ethanol at a dose that did not affect locomotor activity in water-controls, likely acting through 5-HT2 receptors, while Ondansetron, a 5-HT3 antagonist, did not affect this response. These results are consistent with recent findings indicating that one of the mechanisms by which the CRH-1 receptor modulates anxiety depends on sensitization of the 5-HT2 receptor antagonist, and highlight the importance of stress as a modulator of the effects of ethanol during early developmental stages.

A comparison between taste avoidance and conditioned disgust reactions induced by ethanol and lithium chloride in preweanling rats

Adult rats display taste avoidance and disgust reactions when stimulated with gustatory stimuli previously paired with aversive agents such as lithium chloride (LiCl). By the second postnatal week of life, preweanling rats also display specific behaviors in response to a tastant conditioned stimulus (CS) that predicts LiCl-induced malaise. The present study compared conditioned disgust reactions induced by LiCl or ethanol (EtOH) in preweanling rats. In Experiment 1 we determined doses of ethanol and LiCl that exert similar levels of conditioned taste avoidance. After having equated drug dosage in terms of conditioned taste avoidance, 13-day-old rats were given a single pairing of a novel taste (saccharin) and either LiCl or ethanol (2.5 g/kg; Experiment 2). Saccharin intake and emission of disgust reactions were assessed 24 and 48 hr after training. Pups given paired presentations of saccharin and the aversive agents (ethanol or LiCl) consumed less saccharin during the first testing day than controls. These pups also showed more aversive behavioral reactions to the gustatory CS than controls. Specifically, increased amounts of grooming, general activity, head shaking, and wall climbing as well as reduced mouthing were observed in response to the CS. Conditioned aversive reactions but not taste avoidance were still evident on the second testing day. In conclusion, a taste CS paired with postabsorptive effects of EtOH and LiCl elicited a similar pattern of conditioned rejection reactions in preweanling rats. These results suggest that similar mechanisms may be underlying CTAs induced by LiCl and a relatively high EtOH dose.

High ethanol dose during early adolescence induces locomotor activation and increases subsequent ethanol intake during late adolescence

Adolescent initiation of ethanol consumption is associated with subsequent heightened probability of ethanol use disorders. The present study examined the relationship between motivational sensitivity to ethanol initiation in adolescent rats and later ethanol intake. Experiment 1 determined that ethanol induces locomotor activation shortly after administration but not if tested at a later post-administration interval. In Experiment 2, adolescent rats were assessed for ethanol-induced locomotor activation on postnatal Day 28. These animals were then evaluated for ethanol-mediated conditioned taste aversion and underwent a 16-day-long ethanol intake protocol. Ethanol-mediated aversive effects were unrelated to ethanol locomotor stimulation or subsequent ethanol consumption patterns. Ethanol intake during late adolescence was greatest in animals initiated to ethanol earliest at postnatal Day 28. Females that were more sensitive to ethanol's locomotor-activating effects showed a transient increase in ethanol self-administration. Blood ethanol concentrations during initiation were not related to ethanol-induced locomotor activation. Adolescent rats appeared sensitive to the locomotor-stimulatory effects of ethanol. Even brief ethanol exposure during adolescence may promote later ethanol intake.

Ontogeny of ethanol intake in alcohol preferring (P) and alcohol nonpreferring (NP) rats

There is a scarcity of research on ethanol affinity in alcohol-preferring (P) rats before weaning and it is unknown if neonate P rats exhibit ethanol intake preferences comparable to those observed in adult P rats. This study examined ethanol intake in P and alcohol-nonpreferring (NP) rats 3 hr after birth (Experiment 1, surrogate nipple test), at postnatal days (PD) 8, 12, and 18 (Experiment 2, consumption from the floor procedure) and at adolescence (Experiment 3, two-bottle choice test at PD32). The high-preference genotype was readily expressed 3 hr after birth. P neonates drank twice as much ethanol as their NP counterparts. This heightened ethanol preference transiently reversed at P8, reemerged as weaning approached (P18) and was fully expressed during adolescence. These results help to clarify the ontogeny of genetic predisposition for ethanol. Genetic predisposition for higher ethanol intake in P than in NP rats seems to be present immediately following birth.

An acetaldehyde-sequestering agent inhibits appetitive reinforcement and behavioral stimulation induced by ethanol in preweanling rats

Ethanol's motivational consequences have been related to the actions of acetaldehyde, a metabolic product of ethanol oxidation. The present study assessed the role of acetaldehyde in the motivational effects of ethanol on preweanling rats. In Experiment 1 pups (postnatal days 13-14, PD 13-14) were given systemic administration of D-penicillamine (DP, a drug that sequesters acetaldehyde: 0, 25, 50 or 75 mg/kg) before pairings of 1.0 g/kg ethanol and a rough surface (sandpaper, conditioned stimulus, CS). At test, pups given sandpaper-ethanol pairings exhibited greater preference for the CS than unpaired controls, but this preference was not expressed by pups given DP. Pre-training administration of 25 or 50 mg/kg DP completely blocked the expression of ethanol-mediated appetitive conditioning. D-penicillamine did not alter blood ethanol levels. Subsequent experiments revealed that ethanol-induced activation was blocked by central (intra-cisterna magna injections, volume: 1 μl, dose: 0 or 75 μg) but not systemic treatment with DP (0, 25, 50 or 75 mg/kg; ip). These results indicate that: (a) preweanling rats are sensitive to the reinforcing effect of ethanol, and (b) that this effect is associated with the motor activating effect of the drug. These effects seem to be mediated by the first metabolite of ethanol, acetaldehyde.

Naloxone blocks ethanol-mediated appetitive conditioning and locomotor activation in adolescent rats

Age-related differences in ethanol sensitivity could put adolescents at risk for developing alcohol-related problems. Little information exists, however, about adolescent sensitivity to ethanol's appetitive effects and the neurobiological mechanisms underlying ethanol reinforcement during this developmental stage. The present study assessed the role of the opioid system in adolescent rats in an appetitive second-order schedule of ethanol reinforcement and ethanol-induced locomotor stimulation. On postnatal day 32 (PD32), animals were pretreated with the general opioid antagonist naloxone (0.0, 0.75, 1.50, or 2.5 mg/kg) and then given pairings of ethanol (0.0 or 2.0 g/kg, intragastrically) with intraoral pulses of water (conditioned stimulus 1 [CS₁], first-order conditioning phase). CS₁ delivery occurred 30-45 min after ethanol administration when the effect of ethanol was assumed to be appetitive. On PD33, adolescents were exposed to CS₁ (second-order conditioning phase) while in a chamber featuring distinctive exteroceptive cues (CS₂). Preference for CS₂ was then tested. Adolescents given CS₁-ethanol pairings exhibited greater preference for CS₂ than controls, indicating ethanol-mediated reinforcement, but only when not pretreated with naloxone. Blood alcohol levels during conditioning were not altered by naloxone. Experiment 2 revealed that ethanol-induced locomotor activation soon after administration, and naloxone dose-dependently suppressed this stimulating effect. The present study indicates that adolescent rats are sensitive to ethanol's reinforcing and locomotor-stimulating effects. Both effects of ethanol appear to be mediated by endogenous opioid system activation.

Dopamine receptors modulate ethanol's locomotor-activating effects in preweanling rats

Near the end of the second postnatal week motor activity is increased soon after ethanol administration (2.5 g/kg) while sedation-like effects prevail when blood ethanol levels reach peak values. This time course coincides with biphasic reinforcement (appetitive and aversive) effects of ethanol determined at the same age. The present experiments tested the hypothesis that ethanol-induced activity during early development in the rat depends on the dopamine system, which is functional in modulating motor activity early in ontogeny. Experiments 1a and 1b tested ethanol-induced activity (0 or 2.5 g/kg) after a D1-like (SCH23390; 0, .015, .030, or .060 mg/kg) or a D2-like (sulpiride; 0, 5, 10, or 20 mg/kg) receptor antagonist, respectively. Ethanol-induced stimulation was suppressed by SCH23390 or sulpiride. The dopaminergic antagonists had no effect on blood ethanol concentration (Experiments 2a and 2b). In Experiment 3, 2.5 g/kg ethanol increased dopamine concentration in striatal tissue as well as locomotor activity in infant Wistar rats. Adding to our previous results showing a reduction in ethanol induced activity by a GABA B agonist or a nonspecific opioid antagonist, the present experiments implicate both D1-like and D2-like dopamine receptors in ethanol-induced locomotor stimulation during early development. According to these results, the same mechanisms that modulate ethanol-mediated locomotor stimulation in adult rodents seem to regulate this particular ethanol effect in the infant rat.

Social isolation and stress related hormones modulate the stimulating effect of ethanol in preweanling rats

Preweanling rats are highly sensitive to the locomotor stimulation induced by relatively high ethanol doses. In adult mice this ethanol effect is modulated by stress. The goal of the present study was to analyze the role of stress and corticosterone in the stimulating effect of ethanol in preweanling rats. In Experiment 1 15-day-old rats were separated from the mother during a period of 4h in which subjects remained isolated or paired with a littermate. In a third condition pups remained in the home-cage with the dam. After this isolation period pups were given ethanol (0 or 2.5 g/kg) and were tested in a novel environment. Previous data have shown that a similar period of isolation is enough to increase corticosterone levels in preweanling rats. Experiment 2 evaluated the effect of exogenous administration of corticosterone (0, 3 or 6 mg/kg) along with ethanol, and Experiment 3 tested ethanol-mediated locomotor activation in adrenalectomized preweanling rats. The last experiment aimed to test the role of corticotropic releasing factor 1 (CRF1) receptors in locomotion induced by ethanol in isolated pups. According to our results there is a synergism between stress or corticosterone and ethanol in preweanling rats. The interaction between stress (induced by social isolation) and ethanol seems to be mediated by CRF, since blockade of CRF1 receptors cancelled the effect of ethanol in isolated pups. This study highlights the importance of considering stress as a possible intervening variable in studies evaluating ethanol effects in developing animals when maternal separation is used in the experimental procedure.

Differential role of mu, delta and kappa opioid receptors in ethanol-mediated locomotor activation and ethanol intake in preweanling rats

The opioid system modulates ethanol intake and reinforcement in adult and preweanling rodents. While adult heterogeneous rats normally do not show ethanol-mediated locomotor stimulation, preweanling rats show it quite clearly. We recently observed that naloxone, a non-specific opioid antagonist, attenuated ethanol-induced locomotor activation in preweanling rats. In the present study we tested the role of specific opioid receptors (mu, delta and kappa) in ethanol-mediated locomotor stimulation and ethanol intake. In Experiment 1 13-day-old rats received naloxonazine (mu antagonist: 0, 7.5 or 15 mg/kg), naltrindole (delta antagonist: 0, 2 or 4 mg/kg) or nor-binaltorphimine (kappa antagonist: 0, 2, 4 or 8 mg/kg) before an intragastric administration of ethanol (0 or 2.5 g/kg), and subsequent locomotor activity assessment. In Experiment 2, the same opioid antagonists were administered on postnatal days 13 and 14 before consumption of ethanol (6%), saccharin (0.05%) or distilled water. In Experiment 1 only naloxonazine reduced ethanol-mediated locomotor stimulation. None of the opioid antagonists affected locomotor activity in water controls. In Experiment 2 naloxonazine and naltrindole suppressed ingestion of all the solutions tested. Similar to what has been reported in adult rodents, mu-opioid receptors seem to modulate ethanol-activating effects during early ontogeny. Hence, there seems to be a partial overlap of neurochemical mechanisms involved in the rewarding and stimulating effects of ethanol in preweanling rats. Mu-receptor antagonists reduced both ethanol-induced activity and ethanol intake, but it is unclear whether the latter effect is specific to ethanol or only a reflection of an effect on consummatory behavior generally, since mu and delta receptor antagonists also suppressed ingestion of water and saccharin.

Fetal exposure to moderate ethanol doses: heightened operant responsiveness elicited by ethanol-related reinforcers

BACKGROUND

Prenatal exposure to moderate ethanol doses during late gestation modifies postnatal ethanol palatability and ingestion. The use of Pavlovian associative procedures has indicated that these prenatal experiences broaden the range of ethanol doses capable of supporting appetitive conditioning. Recently, a novel operant technique aimed at analyzing neonatal predisposition to gain access to ethanol has been developed. Experiment 1 tested the operant conditioning technique for developing rats described by Arias and colleagues (2007) and Bordner and colleagues (2008). In Experiment 2, we analyzed changes in the disposition to gain access to ethanol as a result of moderate prenatal exposure to the drug.

METHODS

In Experiment 1, newborn pups were intraorally cannulated and placed in a supine position that allowed access to a touch-sensitive sensor. Paired pups received an intraoral administration of a given reinforcer (milk or quinine) contingent upon physical contact with the sensor. Yoked controls received similar reinforcers only when Paired pups activated the circuit. In Experiment 2, natural reinforcers (water or milk) as well as ethanol (3% or 6% v/v) or an ethanol-related reinforcer (sucrose compounded with quinine) were tested. In this experiment, pups had been exposed to water or ethanol (1 or 2 g/kg) during gestational days 17 to 20.

RESULTS

Experiment 1 confirmed previous results showing that 1-day-old pups rapidly learn an operant task to gain access to milk, but not to gain access to a bitter tastant. Experiment 2 showed that water and milk were highly reinforcing across prenatal treatments. Furthermore, general activity during training was not affected by prenatal exposure to ethanol. Most importantly, prenatal ethanol exposure facilitated conditioning when the reinforcer was 3% v/v ethanol or a psychophysical equivalent of ethanol's gustatory properties (sucrose-quinine).

CONCLUSIONS

The present results suggest that late prenatal experience with ethanol changes the predisposition of the newborn to gain access to ethanol-related stimuli. In conjunction with prior literature, this study emphasizes the fact that intrauterine experience with ethanol not only augments ethanol's palatability and ingestion, but also facilitates the acquisition of response-stimulus associations where the drug acts as an intraoral reinforcer.

Binge ethanol exposure in late gestation induces ethanol aversion in the dam but enhances ethanol intake in the offspring and affects their postnatal learning about ethanol

Previous studies show that exposure to 1 or 2g/kg of ethanol during the last days of gestation increases ethanol acceptance in infant rats. We tested whether prenatal exposure to 3g/kg, a relatively high ethanol dose, generates an aversion to ethanol in both the dam and offspring, and whether this prenatal experience affects the expression of learning derived from ethanol exposure postnatally. The answer was uncertain, because postnatal administration of a 3-g/kg ethanol dose induces an aversion to ethanol after postnatal day (PD) 10 but increases ethanol acceptance when administered during the first postnatal week. In the present study, pregnant rats received intragastric administrations of water or ethanol (3g/kg) on gestation days 17-20. On PDs 7-8 or 10-11, the offspring were administered water or ethanol (3g/kg). Intake of ethanol and water, locomotor activity in an open field, and ethanol odor preference were evaluated in the pups, whereas the mothers were evaluated in terms of ethanol intake. Results indicated an aversion to ethanol in dams that had been administered ethanol during gestation, despite a general increase in ethanol intake observed in their pups relative to controls. The prenatal ethanol exposure also potentiated the increase in ethanol intake observed after intoxication on PDs 7-8. Ethanol intoxication on PDs 10-11 reduced ethanol consumption; this ethanol aversion was still evident in infant rats exposed prenatally to ethanol despite their general increase in ethanol intake. No effects of prenatal ethanol exposure were observed in terms of motor activity or odor preference. It is concluded that prenatal exposure to ethanol, even in a dose that induces ethanol aversion in the gestating dam, increases ethanol intake in infant rats and that this experience modulates age-related differences in subsequent postnatal learning about ethanol.

Opioid antagonists block the acquisition of ethanol-mediated conditioned tactile preference in infant rats

It has been difficult to find conditioned preference for tactile cues paired with ethanol intoxication in rats. Toward understanding the ontogeny of ethanol reinforcement, we aimed at establishing a simple and reliable procedure for (1) assessing primary appetitive conditioning to ethanol in infant rats and (2) discerning the role the opioid system plays in ethanol-mediated conditioning at this age. Experiment 1 determined the parameters (i.e., dose, interval of conditioning) for assessing ethanol-mediated conditioning. Pups were then trained with differential Pavlovian conditioning (Experiments 2 and 3) in which ethanol intoxication (1.0-2.0 g/kg, intragastrically or intraperitoneally delivered) was paired with a tactile stimulus (sandpaper) while an alternative texture signaled the absence of ethanol's effects. Unpaired control conditions were also used. Tactile preferences were assessed after two conditioning sessions. Paired rats spent significantly more time on sandpaper than unpaired controls, an effect that was greater after intragastric administration of 1.0 than 2.0 g/kg ethanol. This effect was replicated in Experiments 4a and 4c and found to be inhibited by pretreatment with general (naloxone [NAL]) or specific (d-Pen-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH2 [CTOP] and naltrindole) opioid antagonists. Blood ethanol levels at conditioning were not altered by NAL (Experiment 4b). The study outlines a procedure that reveals appetitive conditioning to ethanol by infant rats. The results are discussed in terms of a potential ethanol-induced activation of the endogenous opioid system during the onset of the intoxication process.

Assessing appetitive, aversive, and negative ethanol-mediated reinforcement through an immature rat model

The motivational effects of drugs play a key role during the transition from casual use to abuse and dependence. Ethanol reinforcement has been successfully studied through Pavlovian and operant conditioning in adult rats and mice genetically selected for their ready acceptance of ethanol. Another model for studying ethanol reinforcement is the immature (preweanling) rat, which consumes ethanol and exhibits the capacity to process tactile, odor and taste cues and transfer information between different sensorial modalities. This review describes the motivational effects of ethanol in preweanling, heterogeneous non-selected rats. Preweanlings exhibit ethanol-mediated conditioned taste avoidance and conditioned place aversion. Ethanol's appetitive effects, however, are evident when using first- and second-order conditioning and operant procedures. Ethanol also devalues the motivational representation of aversive stimuli, suggesting early negative reinforcement. It seems that preweanlings are highly sensitive not only to the aversive motivational effects of ethanol but also to its positive and negative (anti-anxiety) reinforcement potential. The review underscores the advantages of using a developing rat to evaluate alcohol's motivational effects.

Ethanol-mediated aversive learning as a function of locomotor activity in a novel environment in infant Sprague-Dawley rats

Unlike adult heterogeneous rats, infant rats are sensitive to ethanol's locomotor stimulating effects. Susceptibility to this ethanol effect varies as a function of baseline locomotor activity levels. Infant rats with higher baseline activity levels are more sensitive to ethanol's stimulating effects than those with lower baseline activity levels. The present study was designed to analyze susceptibility to ethanol-induced motivational learning in subpopulations of infant heterogeneous rats that differ in baseline activity in a novel environment. On postnatal day 11 (PD 11) baseline locomotor activity was registered and infants were divided into high and low responders (HR, LR). In Experiment 1, pups were trained in a procedure of conditioned taste aversion employing ethanol (0.0, 0.5 or 2.5 g/kg) as unconditioned stimulus (US) and saccharin as conditioned stimulus. In Experiment 2 the same procedure was employed with LiCl (0.0, 0.25 or 0.5% of body weight of a 0.3 M LiCl solution) as US. HR were more resistant to the aversive effects of ethanol than LR while magnitude of LiCl-induced conditioned taste aversion was similar in HR and LR. These results suggest the possibility of early detection of subpopulations of rats with differential sensitivity to ethanol's effects.

Ethanol induces locomotor activating effects in preweanling Sprague-Dawley rats

Abuse of drugs exerts biphasic motor activity effects, which seem to be associated with their motivational effects. In the case of ethanol, heterogenous rat strains appear to be particularly sensitive to the sedative effects of the drug. In contrast, ethanol's activating effects have been consistently reported in rats genetically selected for ethanol affinity. Heightened ethanol affinity and sensitivity to ethanol's reinforcement are also observed in nonselected rats during early ontogeny. In the present study, we examined psychomotor effects of ethanol (1.25 and 2.5 g/kg) in 8-, 12-, and 15-day-old pups. Motor activity in a novel environment was assessed 5-10 or 15-20 min following drug treatment. Rectal temperatures and latency to exhibit the righting reflex were recorded immediately after locomotor activity assessment. Ethanol exerted clear activating effects at 8 and 12 days of age (Experiments 1a and 1b) and to a lesser extent at 15 days. At this age, ethanol enhanced locomotor activity in the first testing interval (Experiment 1b) and suppressed locomotion at 15-20 min (Experiment 1a). Ethanol-mediated motor impairment was more pronounced in the youngest group (postnatal day 8) than in the older ones. Blood ethanol concentrations were equivalent in all age groups. The present study indicates that preweanling rats are sensitive to ethanol's stimulating effects during the second postnatal week, and suggest that specific periods during early ontogeny of the rat can provide a valuable framework for the study of mechanisms underlying ethanol's stimulation and reinforcement effects.

Novelty modulates the stimulating motor effects of ethanol in preweanling rats

During early ontogeny heterogeneous rats are sensitive to ethanol's stimulating effects. In adulthood locomotor activity in a novel environment is a valuable predictor of acute sensitivity to the activating effects of various drugs, including ethanol. Environmental novelty modulates response to ethanol and other drugs in adult rats. The present study analyzed the role of novelty in the acute locomotor response induced by ethanol earlier in development, during the preweanling period, a stage characterized by enhanced sensitivity to ethanol's reinforcing effects. In Experiment 1 we evaluated the predictive value of baseline locomotor activity upon ethanol-induced locomotor effects in 12-day-old rats. In Experiment 2 we tested whether repeated familiarization with the testing environment would reduce the stimulating effects induced by ethanol on postnatal day 12. Individual differences in response to an inescapable novel environment significantly predicted the locomotor activating effects of ethanol, but not other acute effects of the drug, such as hypothermia, motor impairment or sedation. Behavioral activation induced by ethanol during the preweanling period was attenuated after familiarization with the testing environment, suggesting that environmental novelty is critical for activating effects of ethanol.